Our invention relates generally to apparatus for data transfer with magnetic storage media such as tape cassettes, and particularly, to an apparatus capable of recovering and processing data from digital tape cassettes or like storage media on which the data have been recorded with two different methods. Typical examples of such different recording methods are NRZI (nonreturn to zero IBM) and write equalization.
Our invention has a close relation with U.S. Pat. application Ser. No. 170,826 filed Mar. 21, 1988, now U.S. Pat. No. 4,843,490 issued Jun. 27,1989, by Tamotsu Haraguchi under the title of "System for Discriminating between Two Tape Assemblies of Different Recording Characteristics to Be Loaded Interchangeably in Data Transfer Apparatus" and assigned to the assignee of the instant application. The contents of this earlier application will be incorporated herein insofar as is necessary for a full understanding of our invention.
NRZI represents one of the familiar magnetic recording methods of digital signals. Write equalization has more recently been suggested as a refinement of NRZI. Reference may be had to the article entitled, "Write Equalization in High Linear Density Magnetic Recording", by Richard C. Schneider in the Vol. 29, No. 26, issue of IBM J. RES. DEVELOP. published in Nov. 1985, for a detailed discussion of write equalization.
Generally, different recording methods demand different circuit configurations for reproduction. For example, NRZI requires a differentiator for reproduction, but write equalization does not. It will nevertheless be desirable or convenient if tape assemblies on which data have been written by different methods can be read by one and the same apparatus.
The above cross reference Haraguchi application teaches an example of such compatible apparatus, with emphasis on the automatic discrimination between tape assemblies of two different recording methods. It also discloses two separate, self contained reproduce circuits for processing the head output signals representative of the data recovered from the two types of tape assemblies, respectively, together with a switch for selectively connecting either of the reproduce circuits to the magnetic head depending upon which of the two types of tape assemblies has been loaded in the apparatus. We object to the provision of the two self contained reproduce circuits because they add to the cost of the compatible apparatus. Some elements of the reproduce circuits could be used in common for processing the head output signals representative of both kinds of data.